The present invention relates to a drill control method and apparatus, and, more particularly, to a method and apparatus for controlling drilling operations when holes are drilled through a composite workpiece which is made up of a plurality of materials of different machining properties.
Ordinarily, holes are drilled by rotating and feeding a drill at a constant speed. However, in the case where holes are drilled through a composite workpiece made of a plurality of laminated materials of different machining properties, a drilling condition suitable for one of the materials may not be suitable for another material, and boundary-layer peeling or chipping phenomena tend to occur thereby resulting in deterioration of the quality of the drilled holes. In addition, the usable life of the drill tends to be short, and selection of the drilling conditions is extremely difficult.
In order to eliminate the difficulties described above, there has been developed a drilling control method wherein drilling conditions, such as the feed speed and the rotational speed are set beforehand, which are suitable for each material; each material is drilled constantly under the set conditions; the terminating point of drilling of the material is detected from the time required or from an amount of the speed of the drill; the drilling condition is then changed to another one suitable for the material of the next layer and the drilling operation is carried out continuously by changing the drilling conditions one after the other as described above.
For controlling the rotational speed of the drill, there has been proposed a method for controlling the spindle speed for the drill (for instance, refer to Japanese Patent Laid-Open (KOKAI) No. 274843/1986). In this method, a variable speed motor is used for driving the spindle, and this motor is controlled based on a difference between a command speed and an actual speed.
For monitoring the cutting conditions, there has been proposed a device for controlling the machining condition (Japanese Patent Laid-Open (KOKAI) No. 15060/1987). In this device, a load cell is interposed between a chuck and a mounting surface therefor. The load cell detects the thrust force applied to the drill and delivers a voltage corresponding to the thrust. When the voltage exceeds a predetermined value, the terminating point of the drilling operation is detected without requiring any other positioning.
However, even if the drill feeding speed is controlled according to the method disclosed in Japanese Patent Laid-Open (KOKAI) No. 274843/1986, and the completion of the drilling of one material is determined by the device disclosed in Patent Laid-Open 15060/1987, the process becomes too complicated because multiple drilling conditions equal to that of the number of materials must be set beforehand. Furthermore, the feed speed and the rotational speed of the drill must be varied in accordance with the thickness of each material.
In addition, the drilling condition even for the same material must be varied according to the thrust force applied to the drill for preventing the boundary layer from peeling and for improving the quality of the drill hole. Optimum control of the drill is difficult under such conditions. The known method does not provide such operations.